This is a question that has various replies and opinions (both in official and marketing specifications, discussions etc.). Is there a clear answer for that - a fact ? Does Thinkpad T520 have SATA II (3GB/s) or SATA III (6 GB/s), i. e. does it support the full read/write speed

SATA III:
read 550 MB/s
write 520 MB/s

or just

SATA II:
read 280 MB/s
write 260 MB/s

T520 has SATA III for the internal drive port and the ultrabay port.

I think the confusion may come from the fact that Lenovo does not currently sell the machine with a SATA III SSD drive as an option, only SATA II SSD drive. I know my T420 came with a SATA II mechanical drive, so, while I can't say for sure, it would seem the same holds true for the mechanical drives (e.g., SATA II not SATA III).

Yup SATA III confirmed, here is a benchmark I just did.